I. Field of the Invention
This invention relates to devices used for testing for the presence of specific organic compounds in fluids and, in particular, to devices of this kind which exhibit a visible colour change when the specific compounds are detected.
II. Summary of the Prior Art
The testing of fluids, particularly body fluids, for the presence of specific biological or synthetic materials is becoming an increasingly important part of scientific procedures, particularly medical diagnosis and treatment. Such testing is often carried out in laboratories which employ sophisticated and expensive equipment. However, this is not only undesirable from the point of view of the expense, but is often time consuming and requires numerous different samples to be collected in the same place, thus giving rise to the possibility of errors in identifying the origins of the samples.
In order to avoid these disadvantages, there is a growing demand for simple but reliable tests that can be carried out at the point of origin of the samples, for example in a doctor's office, by a patient at home or at any other convenient location. A variety of tests of this type are already commonplace, e.g. it is possible to measure the sugar content of urine by observing a colour change of an absorbent paper strip dipped into the fluid. However, tests for other biological products are often difficult to simplify in this manner, and researchers have been turning to less obvious physical and chemical phenomenona for incorporation into such test procedures, particularly when such phenomenona produce a readily observable change of appearance of an item.
For example, Sagax Instrument AB of Sweden were awarded U.S. Pat. No. 4,558,012 on Dec. 10, 1985 for a "Method and Member for Detecting and/or Measuring the Concentration of a Chemical Substance". In the preferred form, the detection device comprises a thin layer of SiO.sub.2 on a carrier wafer, and a layer of detection reactant or counter reactant (e.g. a layer of an antibody) on the SiO.sub.2 layer. Interposed between the SiO.sub.2 layer and the carrier wafer is at least one additional dielectric layer. The thicknesses of the respective layers are such that interference colours are observable and, when a material to be detected (e.g. to be detected (e.g. an antigen) is trapped as a thin layer by the detection reactant or counter reactant, the interference effect is varied and a colour change is produced.
The problem with this type of device is that the interference colours are not very noticeable and the multi-layer structure is difficult and expensive to produce.
It is known that strong interference colours are produced when certain metals (e.g. Ta) are anodized at high voltages. The anodization causes a thin barrier film of metal oxide to grow at the metal surface and the thickness of the film is such that reflections from the surface of the film and reflections from the underlying metal interfere and generate highly visible colours. Structures of this type are candidates for diagnostic devices because the observed colour is highly dependent on the thickness of the transparent film and small changes in thickness can produce noticeable colour changes. This phenomenon was suggested for use in diagnostic devices by Adams, Kings, Fischer and Vroman in the Journal of Immunological Methods 3(1973) 227-232. In this case, Ta was sputtered onto glass, the Ta was anodized and a bronze colour was observed. The anodized Ta was then coated with a protein and exposed to an antigen-antiserum mixture. The colour changed to reddish purple when a monolayer of antigen was absorbed and this colour changed to deep violet when covered with antibody.
Despite the apparent success in applying anodic interference colours to diagnostic devices reported above, we have found that the colour changes produced in such structures by the adsorption of thin organic layers are not readily observable and are difficult to utilize in practice
Accordingly, there is a need for improved structures capable of exhibiting noticeable colour changes when coated with thin organic films.